1. Field Of The Invention
The invention relates to flotation columns, and particularly to flotation columns for use in the concentration of mineral ores by froth flotation.
2. Reference To Prior Art
Froth flotation involves conditioning an aqueous slurry of a mixture of mineral and gangue particles with one or more flotation agents to promote flotation of either the mineral or the gangue constituents of the slurry when the slurry is aerated. The slurry is aerated by injecting air bubbles which tend to become attached to hydrophobic particles in the slurry, thereby causing these particles to rise to the surface of the body of slurry and form a froth fraction which overflows or is withdrawn from the flotation column. Hydrophilic particles remain in the aqueous phase and are removed therewith.
Column flotation is becoming widely used in a variety of mineral and coal cleaning operations, to separate various mineral species from impurities based on their different surface properties. Coal in particular can be readily separated from impurities by froth flotation, due to its naturally hydrophobic nature. Flotation columns are particularly effective for separating coal particles finer than approximately 0.5 millimeters, and considerably finer particles of other minerals.
Flotation columns for accomplishing froth flotation typically consist of a relatively tall, vertical column. Air is injected at the bottom, a slurry is introduced at approximately the midpoint and fresh wash water is typically injected at the top. As air bubbles rise through the column, hydrophobic particles attach and are carried up to the surface of the slurry and into the froth, while hydrophilic gangue particles settle to the base of the column and are removed in the tailings product. The solids-laden air bubbles forming the layer of froth are flushed with wash water to remove misplaced gangue particles, and the gangue particles return to the aqueous phase. A mixture of mineral particles and froth is removed as clean product from the top of the column.
Among major factors which reduce the efficiency of flotation columns are (a) vertical mixing of the slurry and (b) inadequate separation of gangue particles resulting in misplacement into the froth. In separating coal for example, vertical mixing causes gangue to be carried up into the froth layer, and coal particles to be carried down into the aqueous phase at the base of the column. Misplacement of gangue particles into the froth can occur by weak attachment to bubbles or mechanical entrapment by coal particles attached to the air bubbles. Reducing these vertical mixing and misplacement effects improves the separation selectivity between coal and gangue particles, so that both the froth product and the tailings product are more nearly pure, and both the quality and the recovered quantity of the coal are increased.
Flotation columns are typically 30-50 feet tall. Columns of such height minimize the undesirable effects of vertical mixing, but such height makes the columns difficult to install and operate. For example, rising air bubbles carry a significant amount of water upward, producing a vertical pumping action. Also, coarse air bubbles greater than 1 centimeter in diameter occasionally pass through the column and produce a churning action which disrupts the froth layer. Such vertical pumping and churning actions make the performance of relatively tall columns somewhat erratic.
It is known to employ internal baffles to reduce vertical mixing and increase particle-bubble contact. U.S. Pat. Nos. 4,028,229, 4,066,540, 4,450,072 and 4,851,036 disclose columns having internal baffles. However, there previously has been no convenient means to adjust the open area or vertical spacing of baffling in columns to accomodate different slurry compositions, different minerals or coals, or different slurry flow rates. Nor has there been a convenient means to retrofit baffling into existing unbaffled columns.